PrepTest 112, Section 3, Question 19
My father likes turnips, but not potatoes, which(15 more words not shown)
The flawed reasoning in the(12 more words not shown)
- AThis book is(17 more words not shown)
- BAlthough this recently(28 more words not shown)
- CAll ornate buildings(23 more words not shown)
- DErica enjoys studying(24 more words not shown)
- EPeople who do(23 more words not shown)
Reading the argument
Dad isn't a potato lover. How can his preferences show us an exception among potato lovers?
The rule the author tries to reject is that everyone who likes potatoes also likes turnips. To disprove it, we'd need someone who likes potatoes but dislikes turnips. Dad has exactly the other combination: he likes turnips and dislikes potatoes. He isn't in the group the rule starts with.
What we’re looking for
His preferences would refute a different rule—that everyone who likes turnips also likes potatoes. But the author switches the groups. For the parallel flaw, keep the direction straight: an example has one property and lacks another, then is wrongly used to reject a rule about things that have the property it lacks. The reason Dad dislikes potatoes adds color without changing that mistake.
Working the answers
- A
The book is expensive but isn't a paperback, which initially resembles Dad liking turnips but not potatoes. However, the conclusion denies that even some paperbacks are expensive. That amounts to saying no paperbacks are expensive.
The original rejects an all claim: not every potato lover likes turnips. It does not deny every overlap between the groups. A reaches a different kind of conclusion, so it doesn't match despite the similar example. To match, it would need to reject the claim that all paperbacks are expensive on the basis of this nonpaperback.
- BCorrect
Having more than 75 pages corresponds to liking turnips. Being a novel corresponds to liking potatoes. This work has the first property but lacks the second, just like Dad.
Then B tries to reject the rule that all novels have more than 75 pages. But the example isn't a novel. It doesn't tell us about an exception among novels any more than Dad tells us about an exception among potato lovers. We would need a novel with 75 pages or fewer to refute that rule. The actual example has the wrong combination of properties, matching the original mistake.
- C
This applies a stated rule to a case that meets it. All ornate buildings are from before the twentieth century; this house is ornate; therefore it is from before that century. Nothing is presented as an outside example refuting an all claim. The conclusion follows from the given facts.
- D
This is the tempting one because it copies the language of preferences and gives a reason for the disliked subject. But Erica is a genuine exception to the rule D rejects. She enjoys physics and doesn't enjoy pure mathematics, so she disproves the claim that everyone who enjoys physics enjoys mathematics.
Our original would need to reject the rule starting with turnip lovers to work the same way. Instead, it rejects the rule starting with potato lovers. D keeps its groups in the right order and makes a valid point, so its familiar surface wording hides a different structure.
- E
This gives a general rule from doing your own oil changes to being a car fanatic. It then moves from the neighbors being fanatics to claiming they do their own oil changes. They might express their interest in cars in other ways. That is an unsupported application of the rule in the other direction, not an attempt to refute a rule with an example outside its starting group.
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